Vortices produced by swirl-mixing grids in nuclear fuel assembly: a wind-tunnel experiment

Longitudinal vortices produced by a swirl-mixing grid are experimentally explored in an upscaled model of nuclear fuel assembly. The flow is mapped using particle image velocimetry in several planes downstream of the grid. The flow, an isothermal flow geometrically similar to that in one of the standard nuclear reactors, is compared between basic grids, swirl grids and the case without fuel rods, allowing for a link to previous studies of longitudinal vortex lattices. Individual vortices are recognised using a custom-made algorithm. Analysis of vortices shows that the meandering is enhanced by the presence of fuel rods and by the presence of an upstream swirl grid. The vortex core radii do not grow in the constrained case. There is a weak anticorrelation between the vortex velocity and the actual meandering amplitude. The neighbouring vortices show a weak correlation in their circumferential velocities or energies, but they do not display any significant correlations of positions or meandering amplitudes, cutting down any hypothetical “vortex dancing”.